1. Field of the Invention
The present invention relates to printers. More particularly, the present invention relates to ink jet printers.
2. General Background of the Invention
Inkjet print heads require well-controlled substrate temperature to maintain a consistent ink viscosity and jetting performance. Previous designs include a temperature sense resistor (TSR) integrated into the heater chip to monitor the substrate temperature. The chip also has designated resistor elements to heat the substrate as necessary. The resistor elements may have dedicated power FETs to control the substrate heater resistors, as in Lexmark's U.S. Pat. No. 6,102,515 (incorporated herein by reference). Some designs may use the inkjet resistors themselves for substrate heating, if the on-time is less than the bubble nucleation threshold, as practiced by Hewlett-Packard. The printer control unit periodically monitors the temperature sense resistor to determine the substrate temperature. Then the control unit turns the substrate heaters on and off, accordingly, to maintain the proper substrate temperature for optimum jetting performance.
The temperature sense resistor value follows the equations:RT=Rambient*(1+(α*(T−Tambient)))Rambient=RS ambient*(L/W)                where R=resistance of the TSR        α=temperature coefficient of resistivity, (Ω/° C.),        T=temperature, (° C.), and        Rs=sheet resistance, (Ω/□).        L=length of the TSR material, (μm)        W=width of the TSR material, (μm)Based on this knowledge, the TSR is selected to have a large positive temperature coefficient (α) and a large resistance (R). In the past, the resistor material has typically been chosen to be a metal (AlCu). While metal may have a relatively large positive temperature coefficient, its TSR design is limited by the ability to route enough metal around the chip to get a high enough resistance for easy temperature change detection (see metal resistor 20 in inkjet print head chip 220 in FIG. 1). Typical TSR resistances have been 500–1000 ohms. A 500-Ω TSR, for example, will have a resistance change of approximately 2 Ω/° C. A 1000-Ω TSR, for example, will have a resistance change of approximately 3.5 Ω/° C. Metal TSRs are also limited by the wide tolerance range that can occur. The only way to increase the resistance of a metal TSR is to make the trace longer or the width smaller. Making the TSR longer, takes up silicon area. Making the TSR width smaller, widens the tolerance band, due to process variations in width. For example, a TSR 2 μm (2 microns) wide, with 0.1 μm over-etch on each side will be 1.8 μm wide, a loss of 10% of drawn width. A TSR 20 μm wide, with 0.1 μm over-etch on each side will be 18.8 μm wide, of loss of 1% of drawn width. These are some of the issues currently involved with metal TSR designs.        
The following U.S. patents, and all patents mentioned herein, are incorporated herein by reference:    U.S. Pat. Nos. 6,450,622; 6,443,558; 6,441,680; 6,382,758; 6,336,713; 6,171,880; 6,102,515; 5,300,968; 5,136,305.
U.S. Pat. No. 6,336,713 discloses a thermal inkjet printhead which uses metal silicon nitride resistors as heaters. This patent mentions that resistors having high bulk resistivity are desirable for use in thermal inkjet printing units, and that the resistors disclosed therein have high bulk resistivity (see column 8, lines 29).
U.S. Pat. No. 6,443,558 discloses an inkjet printhead having a thermal bend actuator with a separate titanium nitride heater element. It includes N-well transistors (see column 15).
U.S. Pat. No. 6,171,880 discloses a meandering polysilicon heater mounted on an IC CMOS chip. See column 4, lines 12–18 and 34–41, and column 5, lines 7–36 (fabricated in a CMOS N-well operation).
U.S. Pat. No. 6,382,758 discloses an inkjet printhead having TSRs 14 (see column 3, lines 1–5).
U.S. Pat. No. 6,450,622 discloses a print head with a semiconductor substrate that has an N-well layer, but uses TaAl resistors (see column 3, lines 6–7 and 44–46).
U.S. Pat. No. 5,136,305 discloses controlling heat to ink reservoirs for inkjet printheads using temperature sensitive resistors (see column 4, lines 30–38).
U.S. Pat. No. 5,300,968 discloses a lightly n-doped resistor or a heavily n+doped polysilicon resistor (both of which have high sheet resistance and high temperature coefficient of resistance) in a temperature compensating circuit in an inkjet printhead (see column 5, line 65 through column 6, line 30).
U.S. Pat. No. 6,441,680 discloses a CMOS reference voltage generator using p-type and n-type CMOS transistors. It discusses temperature dependence of these transistors (see, for example, column 4, lines 8–20).